[Server-devel] jffs2 vs ubifs vs ext4 space-efficiency question

[Adam Holt]

Any ideas why Release 13.2.6 placed on ext4 instead of jffs2 appears to
almost double in size?

Starting with 13.2.6 on XO-1 on jffs2 per usual, "du -s /* | sort -nr"
yields these contents for a stock/vanilla XO-1:

   1070992 /usr
   239060  /home
   117623  /var
   18780   /etc
   15382   /boot
   8735/versions
   1032/run
   8/tmp
   ...

Naively summing up the above numbers gives a figure of almost 1.5GB, and
yet we all know the XO-1 is limited to 1.0GB :-)  Is jffs2 somehow
compressing the above files by about 2X, which are clearly contained within
769MB as shown in "df -hT" below?

   Filesystem  Type  Size  Used  Avail   Use% Mounted on
   mtd0   jffs2  1.0G  769M  256M  76%  /
   ...

Basic tests putting 13.2.6 on an SD card with ext4 show a rough doubling in
size with the same files (du's ~1.5GB of files indeed sum to about 1.5GB
when listed using df), which I presume indicates ext4 dispenses with
compression, unlike the more space-efficient to jffs2 (and presumably
ubifs) etc designed for flash/SD devices ?

Apologies for my naivete: Would any ext4 partitioning options (or ubifs
instead) offer any functionality similar to this apparently huge
compression win offered by jffs2?  (Given I'm told jffs2 is not appropriate
for the larger SD cards I'm experimenting with, even before we get to
wear-leveling questions!)
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Re: [Server-devel] [support-gang] jffs2 vs ubifs vs ext4 space-efficiency question

[James Cameron]

Yes, jffs2 compresses data.  That's why it is so slow.  That's why SD
card is faster than it should be otherwise.

No, there's no commonly used compression for ext4.

It would be a performance tradeoff; compression takes time and power.

On Tue, Dec 22, 2015 at 04:24:48PM -0500, Adam Holt wrote:
> Any ideas why Release 13.2.6 placed on ext4 instead of jffs2 appears to almost
> double in size?
> 
> Starting with 13.2.6 on XO-1 on jffs2 per usual, "du -s /* | sort -nr" yields
> these contents for a stock/vanilla XO-1:
> 
>    1070992 /usr
>    239060  /home
>    117623  /var
>    18780   /etc
>    15382   /boot
>    8735    /versions
>    1032    /run
>    8    /tmp
>    ...
> 
> Naively summing up the above numbers gives a figure of almost 1.5GB, and yet 
> we
> all know the XO-1 is limited to 1.0GB :-)  Is jffs2 somehow compressing the
> above files by about 2X, which are clearly contained within 769MB as shown in
> "df -hT" below?
> 
>    Filesystem  Type  Size  Used  Avail   Use% Mounted on
>    mtd0   jffs2  1.0G  769M  256M  76%  /
>    ...
> 
> Basic tests putting 13.2.6 on an SD card with ext4 show a rough doubling in
> size with the same files (du's ~1.5GB of files indeed sum to about 1.5GB when
> listed using df), which I presume indicates ext4 dispenses with compression,
> unlike the more space-efficient to jffs2 (and presumably ubifs) etc designed
> for flash/SD devices ?
> 
> Apologies for my naivete: Would any ext4 partitioning options (or ubifs
> instead) offer any functionality similar to this apparently huge compression
> win offered by jffs2?  (Given I'm told jffs2 is not appropriate for the larger
> SD cards I'm experimenting with, even before we get to wear-leveling
> questions!)

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> support-g...@lists.laptop.org
> http://lists.laptop.org/listinfo/support-gang


-- 
James Cameron
http://quozl.netrek.org/
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Re: [Server-devel] [support-gang] jffs2 vs ubifs vs ext4 space-efficiency question

[Martin Langhoff]

On Tue, Dec 22, 2015 at 4:28 PM, James Cameron  wrote:
> Yes, jffs2 compresses data.  That's why it is so slow.  That's why SD
> card is faster than it should be otherwise.

+1 on James reply. Also a quick note: AIUI, jffs2 gets much of its
storage advantage from better packing of directory structures &
metadata.

The overall compression savings include these, and look great. The
files contents are not _that_ compressible :-)



m
-- 
 martin.langh...@gmail.com
 -  ask interesting questions
 - don't get distracted with shiny stuff  - working code first
 ~ http://docs.moodle.org/en/User:Martin_Langhoff
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